The lead-acid battery is an electrochemical storage battery generally comprising a positive plate, a negative plate, and an electrolyte comprising aqueous sulfuric acid. The plates are held in a parallel orientation and electrically isolated by porous separators to allow free movement of charged ions. The positive battery plates contain a current collector (i.e., a metal plate or grid) covered with a layer of positive, electrically conductive lead dioxide (PbO2) on the surface. The negative battery plates contain a current collector covered with a negative, active material, which is typically lead (Pb) metal.
During discharge cycles, lead metal (Pb) supplied by the negative plate reacts with the ionized sulfuric acid electrolyte to form lead sulfate (PbSO4) on the surface of the negative plate, while the PbO2 located on the positive plate is converted into PbSO4 on or near the positive plate. During charging cycles (via an electron supply from an external electrical current), PbSO4 on the surface of the negative plate is converted back to Pb metal, and PbSO4 on the surface of the positive plate is converted back to PbO2. In effect, a charging cycle converts PbSO4 into Pb metal and PbO2; a discharge cycle releases the stored electrical potential by converting PbO2 and Pb metal back into PbSO4.
Lead-acid batteries are currently produced in flooded cell and valve regulated configurations. In flooded cell batteries, the electrodes/plates are immersed in electrolyte and gases created during charging are vented to the atmosphere. Valve regulated lead-acid batteries (VRLA) include a one-way valve which prevents external gases entering the battery but allows internal gases, such as oxygen generated during charging, to escape if internal pressure exceeds a certain threshold. In VRLA batteries, the electrolyte is normally immobilized either by absorption of the electrolyte into a glass mat separator or by gelling the sulfuric acid with silica particles.
One major problem with existing lead-acid batteries is their low cycleability at high rate charge/discharge conditions required for advanced applications such as hybrid electric vehicles and distributed storage. The main failure mode in these operating conditions is called “negative plate sulfation”, which is a term used to describe the phenomenon of kinetically irreversible formation of lead sulfate (PbSO4) crystallites. Ideally during each charge/discharge cycle all the lead sulfate on the negative plate is reversibly converted to lead and then back to lead sulfate. However, in reality this is not the case and during each cycle more and more lead sulfate is irreversibly formed in the negative plate. The formation of increased amounts of lead sulfate leads to several undesirable effects: the conductivity and porosity of the plate is decreased, the accessibility of sulfuric acid to the active phase is hindered and less Pb is available to participate in the discharge process, all this in combination leading to failure of the battery to deliver the required voltage and power. This phenomenon is especially pronounced when fast charge/discharge rates are used.
One known method for reducing the problem of “negative plate sulfation” is to add carbon to the paste used to produce the negative plate, generally as part of an expander formulation comprising barium sulfate, carbon, and a lignosulfonate or other organic material. The carbon increases the electrical conductivity of the active material in the discharged state thereby improving its charge acceptance. An example of such an approach is discussed in “Mechanism of action of electrochemically active carbons on the processes that take place at the negative plates of lead-acid batteries”, Pavlov et al, Journal of Power Sources, 191, 2009, 58-75, in which the effect of adding different forms of carbon at varying levels between 0.2 to 2% by weight of the negative plate paste is studied. The carbon materials investigated are NORIT AZO activated carbon and the carbon blacks VULCAN XC72R, Black Pearls 2000 and PRINTEX® XE2.
Carbon as an additive has been proven to enable high dynamic charge acceptance and improved cycle life of both flooded and VRLA lead-acid batteries. Addition of carbon to the negative plate changes the morphology of the plate, affects the chemical processes of lead sulfate-lead transformations and thus slows down the process of negative plate sulfation in parallel to improving the charge acceptance characteristics of the battery. Carbon blacks are among several types of carbons studied, along with graphite and activated carbons, and have been demonstrated to offer the most promising results in improved cycleability and charge acceptance. Unlike graphite and activated carbons, carbon blacks consist of submicron aggregates comprised of nm size primary particles. This allows for increased conductivity of the negative plate and for high accessibility of their surface area. Carbon black loadings up to 3 wt. % (alone or in combination with other carbons) have been tested in VRLA and flooded batteries (see, for example, U.S. Patent Application Publication No. 2009/0325068). Higher loadings of high surface area carbons are desirable but there are several challenges in the manufacture of the negative plates when high carbon loadings are targeted. Thus, high loadings of carbon require higher amounts of water to be added to the paste and lead to decreased viscosity and problems with pasting the electrodes over the grid. In addition, during cycling, some of the carbon can “shed off” the electrodes and ultimately negatively affect the cycle life of the battery.
Although carbon addition assists in increasing the electrical conductivity of the negative plate in its discharged state, it does not address another issue associated with sulfation and that is the accessibility of the sulfuric acid electrolyte to the active material. Thus, lack of uniform and controlled distribution of sulfuric acid within the negative plate electrode has been identified as one of the main contributing factors to sulfation, particularly with flooded cell batteries.
According to the invention, both the problems of reduced electrical conductivity and sulfuric acid access are addressed by adding a composite particle comprising carbon and silica to the negative plate paste. The carbon acts to improve the electrical conductivity of the active material, while the presence of silica in the composite particle will act as “glue” holding the carbon particles in aggregates which have controlled porosity and stability during the cycling process. In flooded type lead acid batteries the composite carbon silica particle will also assist in minimizing the acid stratification issue observed during cycling due to partial gelling of the electrolyte. Thus the addition of composite carbon-silica particles to the negative electrode of flooded batteries enables formation of an electrode structure where at least partially the sulfuric acid electrolyte is immobilized onto the electrode and issues with acid accessibility and stratification are significantly improved.
U.S. Patent Application Publication No. 2004/0180264 discloses a lead-acid battery comprising a cathode, an anode and an electrolytic solution, wherein into the anode is added an active carbon or a carbon black or a mixture thereof containing at least one simple substance selected from the group consisting of Hf, Nb, Ta, W, Ag, Zn, Ni, Si, Mg, Al, Co, Mo, Cu, V, Mn, Ba, K, Cs, Rb, Sr and Na, or at least one compound thereof. The disclosed battery containing carbon with the above additives as impurities in 10-5000 ppm concentration levels has improved high efficiency charging characteristics. In addition, an anode containing an activated carbon produced from coconut husk containing up to 15,000 ppm of natural impurities (Cu, Mn, Al, Si and K) is stated to improve the high efficiency charging characteristics and charge acceptance. For all examples the loading of the additive metal substance to the carbon does not exceed 15,000 ppm or 1.5 wt. %.